Double-stranded RNA (dsRNA) can induce sequence-specific posttranscriptional gene silencing in many organisms by a process known as RNA interference (RNAi). However, in mammalian cells, dsRNA that is 30 base pairs or longer can induce sequence-nonspecific responses that trigger a shut-down of protein synthesis. Recent work suggests that RNA fragments are the sequence-specific mediators of RNAi (Elbashir et al., 2001). Interference of gene expression by these small interfering RNA (siRNA) is now recognized as a naturally occurring strategy for silencing genes in C. elegans, Drosophila, plants, and in mouse embryonic stem cells, oocytes and early embryos (Cogoni et al., 1994; Baulcombe, 1996; Kennerdell, 1998; Timmons, 1998; Waterhouse et al., 1998; Wianny and Zernicka-Goetz, 2000; Yang et al., 2001; Svoboda et al., 2000). In mammalian cell culture, a siRNA-mediated reduction in gene expression has been accomplished only by transfecting cells with synthetic RNA oligonucleotides (Caplan et al., 2001; Elbashir et al., 2001).